setlocale

Setlocale is a function in the C standard library that sets or queries the program’s current locale for one or more categories. It is declared in locale.h and has the prototype char setlocale(int category, const char locale). The category selects which aspect of localization to affect, such as LC_ALL, LC_CTYPE, LC_NUMERIC, LC_TIME, LC_COLLATE, LC_MONETARY, and LC_MESSAGES; some platforms may expose additional categories and some categories may not be fully controllable at runtime.

The locale parameter specifies the locale to use. If locale is non-null, setlocale attempts to switch the named category (or categories) to that locale and returns a string containing the actual locale name in effect. If locale is NULL, setlocale returns the current locale name for the specified category without changing it. If locale is the empty string "", the environment variables (for example LC_ALL, LC_CTYPE, LANG) are consulted to determine the locale. If the specified locale name is invalid or unsupported, setlocale returns NULL and the processor’s locale for that category is not changed.

Categories control different aspects of localization. LC_ALL applies changes to all categories at once; other categories control specific areas such as character encoding and classification (LC_CTYPE), number formatting (LC_NUMERIC), date and time formatting (LC_TIME), string collation order (LC_COLLATE), monetary formatting (LC_MONETARY), and message translations (LC_MESSAGES). The exact behavior can vary by implementation.

In practice, setlocale affects locale-dependent functions and behavior, including string collation, numeric formatting, monetary formatting, and character classification, by selecting the appropriate locale for the targeted category.